X-ray luminescence computed tomography (XLCT) is an emerging imaging modality, in which x-ray photons excite nanophosphors to emit optical photons that are measured for optical imaging. XLCT combines the high measurement sensitivity of optical imaging and the high spatial resolution of x-ray imaging. For targets in dee tissues, the optical scattering is overcome by introducing x-ray beam size and location into the reconstruction algorithm. We have built an XLCT imaging prototype system, by which we have demonstrated that the imaging resolution is independent of the depth. To further improve the spatial resolution, we have proposed the microscopic x-ray luminescence computed tomography (microXLCT), in which we used fine collimators to form x-ray beams with diameter less than 100 micrometer. While we got promising preliminary results, the microXLCT suffers from low x-ray utilization efficiency as collimator absorbs most x-ray photons. In this proposal, we aim to improve the x-ray photon utilization efficiency and to have a finer x-ray beam by using a polycapillary optic lens mounted on an x-ray tube. The x-ray photon utilization efficiency will be improved by more than 50 times, which will reduce the measurement time substantially to make the system more practical. The finer x-ray beam will improve the imaging spatial resolution to be less than 100 micrometer. We will optimize and evaluate the microXLCT performance with phantom and euthanized mice experiments. This small grant will mainly support the purchase of this specific x-ray tube with the polycapillary lens and labor cost. Other major components in the imaging system, such as an electron multiplying charge coupled device (EMCCD) camera with a cooling system, an x-ray shielding cabinet with safety interlock, and x-ray detector, are already available in my lab for this project. Thus we believe the proposed project fits an NIH R03 grant very well and will also have high significance by providing a powerful and feasible imaging tool for the molecular imaging community.